Treatment of hypophysectomized female rats with FSH stimulated the synthesis of prostaglandins (PG) by ovarian granulosa cells. This dose responsive effect was augmented by exogenous hCG and was observed for both PGF2 alpha and PGE. Injections of hCG and/or PRL had no effect without simultaneous FSH treatment. However, infusion of PRL into FSH-or FSH/hCG-treated animals exerted marked effects on PG synthesis depending upon the circulating levels of PRL achieved. Physiologic replacement doses doubled the amount of PGE synthesized whereas, supraphysiologic doses caused a 90% reduction. This biphasic effect was not seen in PGF2 alpha synthesis, only supraphysiologic doses achieving a 75% suppression. Concomitantly, the precursor for these prostaglandins, arachidonic acid, was shown to be virtually depleted by maintenance of the hyperprolactinemic state. Prostaglandins are extremely potent hormones that act locally and have been shown previously to control the synthesis of ovarian steroids and the ovulatory process itself. The observations suggest that prolactin exerts its effects through the PG cascade and explains, in part, the mechanism by which infertility occurs in hyperprolactinemic women.